An observation of single-molecule fluorescence, a measurement of fluorescence intensity of a specimen having low fluorescence intensity, and so on require a highly accurate apparatus for an observation or a measurement in use of a fluorescence observation measurement apparatus such as a fluorescence microscope.
Auto-fluorescent light from a glass used for a lens of an optical system has been known as one of the causes which deteriorate the accuracy of such apparatus in image observation or measurement.
This auto-fluorescent light is fluorescent light which is emitted by a glass of a material for lens after the glass absorbs a part of excitation light when the excitation light passes through the lens to excite a specimen. This auto-fluorescence light makes a highly accurate observation difficult because a superposition of this auto-fluorescent light and fluorescent light which is emitted from a specimen causes loss of information about a dark portion of a fluorescence image. In fluorescence intensity measurement, the auto-fluorescent light becomes background noise and an accurate measurement for a specimen of low fluorescence intensity becomes difficult. The use of a glass the auto-fluorescence intensity of which is reduced makes it possible to carry out a highly accurate observation or measurement.
Also, if ultraviolet light is used as excitation light in a case where a specimen is a living cell or the like in such a fluorescence observation and/or measurement, the living cell may be damaged. For this reason, for example, visible light with a wavelength of 480 nm is often used.
The arts of a glass the auto-fluorescence intensity of which is reduced (, where the glass is called a low fluorescent glass hereinafter,) include an art of a glass in which an impurity content is reduced, as disclosed in Japanese patent Kokai No. Hei 4-219342 and Japanese patent Kokai No. Hei 11-106233. Also, there is a producing method in which a mix of impurities is reduced in production of a glass, as disclosed in Japanese patent Kokai No. Hei 11-106233.
Japanese patent Kokai No. Hei 4-219342 discloses as a glass with low impurity content a low fluorescent glass which contains As2O3 of 0.05% or less, Sb2O3 of 0.05% or less, V2O3 of 10 ppm or less, CuO of 10 ppm or less, and CeO2 of 1 ppm or less as an impurity.
Also, Japanese patent Kokai No. Hei 11-106233 discloses as a glass with low impurity content a low fluorescent glass which contains platinum of 10 ppm or less as an impurity, does not contain arsenic substantially, and does not contain also antimony substantially. Also, the method disclosed as a producing method is a method of producing a glass in which an amount of the impurity is reduced by inhibiting a mix of platinum from a gas phase and a mix of platinum from an interface between a platinum crucible and glass melt.